The proto-oncogene product Src is a tyrosine kinase whose activity is regulated by other kinases, phosphatases and binding proteins in response to extracellular signals. Deregulated Src is oncogenic. In the previous granting period, we discovered a Src substrate and binding protein, mDabl p80. The mdab1 gene is related to a Drosophila gene, disabled, that is implicated in tyrosine kinase signaling during development of the embryonic central nervous system. Because of the unresolved aspects of how disabled functions in Drosophila, and because mDabl p80 was likely to be involved in essential tyrosine kinase functions in mammals, we studied the expression and structures of mDabl proteins, their biochemical interactions and biological functions. mDabl proteins, made from different mRNAs, are expressed and tyrosine phosphorylated in the developing brain and in hematopoietic cells. The p80 protein binds to other cell proteins and membranes through a PTB domain and phosphotyrosine residues. Thus the protein likely operates on a signal transduction pathway, regulated by, or regulating, tyrosine kinases including Src. Mutations of the mdabl gene by targeted disruption or spontaneous mutations cause abnormal brain development and possibly altered hematopoiesis. Our aims are to learn how mDabl p80 regulates brain development and identify other genes and proteins in the signaling pathway. To this end, we will replace the mdabl gene with altered genes that encode mDabl proteins with changes in the PTB or phosphorylation sites, or both, and thus determine which parts of the molecule are needed for function in vivo and in vitro. We will use genetics and protein biochemistry to test which other genes and proteins are in the mDabl signaling pathway. We will also investigate the roles of mdabl in hematopoiesis and in leukemias caused by the Abl tyrosine kinase. Studies on the mdabl-related gene mdab2 are also proposed. Investigating these signaling proteins may reveal new pathways for regulating cell behavior in response to tyrosine kinase activity.